Ribosome recoding (frameshifting, stop-codon readthrough) is used by many plant-, fungal-, animal- and human-infecting viruses to produce two proteins from a single 5'-end translation initiation site. Most recoding is dependent on a pseudoknot-containing Recoding Structured Element (RSE) positioned just downstream from the stop codon/frameshift site. This exploratory proposal is designed to test the overarching hypothesis that similar recoding events necessary to generate the RdRp by ribosome recoding in Turnip crinkle carmovirus (TCV) and SARS-coV involve similar overall mechanisms. We posit that both viruses require similar alternative basal conformations requiring an additional hairpin and sequences upstream of their RSE and similar long-distance interactions between their RSEs and the 3' and 5' ends of their genomes. This hypothesis is based on finding that TCV contains a stable alternative (basal) structure that disrupts its RSE, and that similar alternative structures are predicted for all coronavirus RSEs examined. In addition, a long-distance RNA-RNA interaction that connects RSEs in plant viruses with their 3' terminus is also conserved in coronavirus RSEs. Furthermore, the RSE sequence in carmoviruses and coronaviruses that has the pairing partner near the 3' end also has a possible pairing partner (7-8 nt for coronaviruses) near their 5' ends. We propose that the 3' end interaction stabilizes the active RSE structure allowing ribosomes to readthrough /frameshift, and that the RSE-5' end interaction occurs when the RSE is in the basal conformation to aid in ribosome recycling following translation termination. In Specific Aim 1, we will investigate the importance of a conformational switch between basal and active structures in the RSE region of TCV using SHAPE RNA structure probing of full-length virus combined with selective mutations. We will also determine if the known long-distance interaction between the TCV RSE bulge loop and 3' terminal sequences stabilizes the active RSE structure. Using single and compensatory mutagenesis, we will also test (with collaborator Dr. Ralph Baric) if a basal conformation exists for the RSE of SARS-coV and if a phylogenetically conserved hairpin loop in the RSE of SARS-coV is involved in a very similar interaction with a 3' sequence. In Specific Aim 2, we will investigate if the RSEs in TCV and SARS-coV play an additional role in ribosome recycling by engaging in a predicted long-distance interaction with the 5' end. We will also investigate if release of this interaction in TC promotes a conformational switch to the RSE active structure for ribosome readthrough. In Specific Aim 3, we will use in-line RNA structure probing of isolated TCV and SARS-coV fragments to investigate structural requirements for the basal and active RSE conformations. The results of these experiments will likely transform current models on ribosome recoding and provide evidence for the importance of ribosome recycling, which has long been lacking in the translation field. It should also, importantly, open up new targets for antiviral agents against viruses that are important human pathogens.